The present invention relates to a method for determining the reliability of state of health parameter values for a battery comprising a plurality of battery cells. The invention is applicable for determining the reliability of state of health parameter values for vehicle batteries, in particularly batteries for hybrid buses or hybrid truck vehicles. However, although the invention will mainly be described in relation to a bus, the invention is of course also applicable for other types of vehicle batteries, such as batteries for cars, industrial construction machines, wheel loaders, etc.
Vehicle batteries are continuously developed in order to e.g. be more durable as well as to function in a broader field of technology, such that the increasing demand from the market can be met. Especially, there is always a desire to reduce emissions from vehicles and there is hence an increased desire to use batteries for propelling the vehicles, either entirely or partly. For example, hybrid buses often have an electric motor which is configured to propel the bus at low speed, and when the speed exceeds a threshold speed limit, an internal combustion engine is started and takes over the propulsion of the vehicle.
The battery is hence arranged to supply electric energy to an external application, and is configured to receive energy in order to be charged. More specifically, the battery comprises a plurality of battery cells which either discharge energy or being charged with energy. The charging of the battery cells can be made when e.g. the vehicle is braking and a generator absorbs energy which is transferred to the battery cells.
One important aspect in the field of batteries is to sufficiently keep track of the status of the battery. It is, for example, important to be aware of e.g. the utilization of the battery in order to know when it is time to replace the battery with a new one. State of Health (SOH) is often used in order to determine aging of the battery as a comparison between a new condition and a deteriorated condition.
U.S. Pat. No. 8,269,502 describes a method for determining the State of Health of a battery. The method described in U.S. Pat. No. 8,269,502 continuously determines cell impedance as well as one or more confidence coefficients that depend on e.g. cell current, temperature or State of Charge.
However, U.S. Pat. No. 8,269,502 relates mainly to the determination of State of Health for a single cell of a battery, and there is hence a need to further improve the process of accurately determine State of Health for a complete battery comprising a plurality of battery cells.
US 2006/0284617 relates to an apparatus for determining a condition parameter of a battery. Amongst other things, US 2006/0284617 relates to state of health of a battery. Voltage, current and temperature signals are calibrated in order to insure that data is accurate.
EP 2 410 346 relates to a method for determining a parameter such as the resistance of at least one accumulator of a battery. EP 2 410 346 determines the absolute uncertainty of the resistance by using standard deviation and of a Student coefficient.
It is desirable to provide a method for determining if the parameters used when calculating state of health are reliable, i.e. if the measured parameters will enable for a substantially correct calculation of a battery state of health.
According to a first aspect of the present invention there is provided a method for determining the reliability of state of health parameter values for a battery comprising a plurality of battery cells, the method comprising the steps of: receiving, for a state of health parameter, a plurality of measured parameter values for the battery;
comparing the measured parameter values with at least one predetermined parameter criterion; and determining that the measured state of health parameter values are reliable if the state of health parameter values fulfil the at least one predetermined parameter criterion.
The wording “state of health parameter values” should in the following and throughout the entire description be interpreted as a parameter value which can be used when calculating the state of health of the battery. The state of health of a battery can be calculated rather differently depending on the specific battery application. Various ways of calculating the state of health for a battery are well known to the person skilled in the art and the following description will hence mainly focus on the description of the different parameters that can be used for calculating the state of health of the battery and not focus on the specific calculation.
Furthermore, it should be readily understood that the present invention relates to determining the reliability of state of health parameter values for a complete battery including a plurality of cells, and not to individual cells of a battery. The battery may include more than e.g. 50 battery cells, such as e.g. 200 cells.
Also, the “predetermined parameter criterion” should be understood as a criterion which can be different for different types of parameters. For example, a predetermined parameter criterion for a temperature parameter is naturally different compared to a predetermined parameter criterion for a voltage parameter. However, the predetermined parameter criterion may also be different for a specific parameter. For example, and as will be described further below, one predetermined parameter criterion may be that a cell temperature may not exceed a certain limit, while another predetermined parameter criterion may be that a temperature difference between the warmest battery cell and the coldest battery cell may not differ too much. Hence, there are two criteria for the temperature parameter. Also, the specific criterion may be set individually by the user and may thus be different depending on the specific application.
One way of measuring the state of health parameter values is to utilize a Kalman filter, also known as Linear Quadratic Estimation. Such methods are well known to the person skilled in the art of measuring parameters of a dynamic system, such as the battery, and will therefore not be described further.
The present invention is based on the insight that by determining if the parameter values for calculating battery state of health are reliable, a correctly calculated state of health can be made. Further, the parameter values which are used when calculating state of health of a battery may for a number of reasons, which will be further described below, not be reliable at certain conditions or states of the battery. The present invention aims at providing a solution where parameters measured at these conditions, or states, are sorted out as non-reliable.
An advantage of the present invention is that if it is determined that the state of health parameters are not reliable, a calculated state of health based on this non-reliable value will not be sufficiently correct and the method according to the present invention thus determines that the calculation should not be executed since the result will not provide a reliable indication of the state of health of the battery. Hence, the present invention provides a method which determines if it is suitable to calculate state of health of the battery with the measured parameter values. By only calculating state of health of the battery when the parameter values for doing so are reliable, a substantially correct calculation of the true state of the battery can be made. Furthermore, with the present invention it is possible to determine if the result from a state of health calculation with the above state of health parameter values will provide a more reliable and correct estimation of the battery state of health compared to what is already available for the user of the battery. Hereby, it may be determined to calculate the battery state of health if it will provide a result which is more correct and reliable than what is already available.
With the present invention, the predictability of, for example, when it is time to replace the battery with a new one will be increased. Hence, the user of the battery will be aware of the state of the battery and does not have to e.g. replace the battery too early or too late. Another advantage is that the operation of the battery can be controlled for optimized capacity for providing electrical capacity at approximately all times. This will in turn increase the fuel efficiency and hence reduce the total operational cost for the owner of the vehicle. Furthermore, the service intervals for calibration/determination of the battery age may also be adapted based on the accurately calculated state of health such that the time period of such intervals may be increased or decreased.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the received plurality of parameter values for the battery are measured when the absolute value of the derivative function dy/dx is above a predetermined threshold limit, where:
y=a measured voltage of the battery; and
x=a calculated state of charge of the battery.
Hereby, it is determined that the calculation of state of health is not executed if the gradient of a voltage-state of charge curve is below a specific value. This is advantageous since if the derivative function is zero, or close to zero, it may be determined that a specific voltage value corresponds to a state of charge value which is either too high or too low in comparison to the “correct” state of charge value of the battery. Also, a small increase or decrease of the battery voltage will, if the gradient is close to zero, result in a rather large increase/decrease of the battery state of charge which therefore makes the use of such values inappropriate when calculating the battery state of health, since the accuracy of the calculated state of charge is not sufficient, which will provide an unreliable calculation of the state of health. The derivative function above is preferably applicable for a so called open cell voltage curve.
It should be readily understood that the person skilled in the art is well aware of how to calculate state of charge of a battery and a description of such calculation is therefore omitted from the description of the present invention. Moreover, the derivative function above may result in either a positive or a negative value, and it is therefore important to analyse the absolute value of such value when determining if the measured value is reliable to use in a state of health calculation. For e.g. an open cell voltage value, the derivative function generally always provides a positive value.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that a mean temperature of the battery cells is within a predetermined temperature range.
If the mean temperature of the battery cells is either too high or too low it may not be suitable to calculate the state of health of the battery since the temperature values may not sufficiently describe the true characteristic of the cells. Hereby, the method determines that it is not suitable to execute a state of health calculation for the battery. The mean temperature of the battery cells may for example be higher than “normal” in case the measurement of the cell temperature is made at a point in time when the battery is charged or discharged. Furthermore, it may be important to determine that the temperature is within a specific range, which range substantially corresponds to a test temperature range when modelling battery state of health. Hence, the temperature range should preferably be approximately the same as the test temperature range such that calculation errors due to temperature differences are reduced.
Also, other state of health parameter values may have a negative impact on a too warm or too cold battery, which thus not only make the temperature parameter value unreliable itself, but also the other state of health parameter values may be considered unreliable if the battery temperature is too high/low. The battery temperature is generally generated by the electric current of the battery. Hence, deviations in temperature may be the result of current flowing through the battery and the measured temperature may thus not provide a value which is accurate or reliable enough. Further, and as described below, the temperature measurement should preferably be made a predetermined time period after the battery has been charged or discharged in order to be sufficiently reliable. In case the temperature measurement is made before the end of the predetermined time period after the charging/discharging of the battery, the temperature may not have “converged” to its true limit, thus making such measured values less accurate/reliable.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the measured parameter values were received a predetermined time period after a previous charging/discharging of the battery was executed.
Hereby, the battery will be allowed to “rest” sufficiently before the calculation is executed, which thus makes the calculated state of health reliable. A time period shortly after a charging/discharging of the battery may provide a too large deviation of the measured parameter for the different cells, and thus not provide a reliable calculation of the state of health. Hence, the battery is not in a steady state.
Furthermore, if the measured parameter value is e.g. the temperature, then it may be determined that the battery is not in a steady state if the temperature tends to increase/decrease during the period when receiving the parameter values.
Accordingly, if the temperature of the battery is increasing or decreasing during the period when receiving the parameter values, then it may be determined that charging/discharging of the battery was executed too close in time from the measurement. However, in cases where the measurement relates to battery state of charge, the time period since the previous state of charge calculation should not exceed a predetermined time period. As described above, the state of charge should be calculated when the derivative function is “high”. Hence, a state of health calculation should be executed within a specific time period after such calculation of the battery state of charge. If the state of health calculation is made after a rather “long” period since the state of charge calculation was made, the state of charge value may not be sufficiently reliable.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the values of the plurality of measured parameter values are within a predetermined range.
Hereby, the spread of the measured values must be within a predetermined range in order to be determined reliable. A battery is often sensitive to high and/or low temperatures and a large spread in temperatures between the different battery cells may thus not provide a sufficiently correct calculation of the battery state of health. The spread between the cells may be for any kind of state of health parameter, such as battery state of charge, battery voltage, battery temperature, etc. The various parameters will be described further below.
According to an example embodiment, the predetermined range may be measured from a mean value of the measured parameter values.
Hereby, it may be determined that a state of health calculation should not be performed if a measured value is deviating too much from a mean value of the measurement.
According to an example embodiment of the present invention, the state of health parameter may be cell temperature of the battery, such that a plurality of measured battery cell temperatures is received.
A battery is often sensitive to high and/or low temperatures, as described above. For example, a large spread in temperatures between the different battery cells may not provide a sufficiently correct calculation of the battery state of health. The temperature of the battery may also be an important parameter to consider when calculating state of health of a battery, since the battery temperature may affect other measured parameters of the battery, such as e.g. the battery voltage.
According to an example embodiment of the present invention, the state of health parameter may be cell voltage of the battery, such that a plurality of measured battery cell voltages is received.
Hereby, a further parameter which may be important when calculating state of health can be used for consideration.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the measured cell voltage of each of the battery cells are below a predetermined upper voltage limit.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the measured cell voltage of each of the battery cells are above a predetermined lower voltage limit.
If the cell voltage value is above a predetermined upper voltage limit or below a predetermined lower voltage limit, the cell having the low or high voltage value may for some reason be damaged. Hereby, calculating battery state of health based on such value may not provide a reliable result of the “true” state of health of the battery. Also, the measured cell voltage may be compared to a deviation from the mean value of the measured cell voltages and if the deviation is too severe, it may be an indication that the measured cell voltage will provide an unreliable state of health value.
Furthermore, if the difference in cell voltage is too severe between the cells, i.e. the range between the lowest cell voltage and the highest cell voltage exceeds a predetermined voltage cell spread range, there may be an unbalance in the battery system which may be caused by the battery having been used for too long without balancing have been performed to the battery, or that the battery has rested for too long without having been used, etc. However, it is difficult to ascertain if there is an unbalance in the battery system or not and the result from a state of health calculation may thus not be considered reliable enough since it is difficult to know that the result of the calculation correspond to the true characteristic of the battery. Hence, the battery cells need to be sufficiently balanced before state of health calculation is executed.
According to an example embodiment of the present invention, the method may further comprise the step of calculating a state of charge value for each battery cell by means of the received parameter values.
Hereby, the measured parameter may be used for calculating the battery state of charge and the method may thus thereafter determine if the calculated state of charge is reliable or not. A further parameter is thus provided for determining if calculation of state of health of the battery should be executed or not.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the calculated state of charge value for each of the battery cells are within a predetermined range.
For the same reasons as described above, if the state of charge of the different cells of the battery is outside a specific range, it may be an indication that a calculation of the state of health of the battery will not provide a sufficiently accurate state of health of the battery, i.e. an unreliable result. It may also be an important aspect for other measured parameters that the state of charge for the different battery cells is at approximately the same level of state of charge.
According to an example embodiment of the present invention, the at least one predetermined parameter criterion may be that the deviation from a mean value of the calculated state of charge values for the battery cells are within a predetermined range.
According to a second aspect of the present invention there is provided a system connectable to a battery comprising a plurality of battery cells, the system comprises a control unit which is configured to: receive, for a state of health parameter, a plurality of measured parameter values for the battery; compare the measured parameter values with at least one predetermined parameter criterion; and determine that the measured state of health parameter values are reliable if the state of health parameter values fulfil the at least one predetermined parameter criterion.
Effects and features of this second aspect are largely analogous to those described above in relation to the first aspect of the present invention.
According to a third aspect of the present invention there is provided a computer program comprising program code means for performing the steps of the above described method when the program is run on a computer.
According to a fourth aspect of the present invention there is provided a computer readable medium carrying a computer program comprising program code means for performing the steps of the above described method when the program is run on a computer.
Effects and features of the third and fourth aspects of the present invention are largely analogous to those described above in relation to the first aspect of the present invention.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. For example, the various parameter criteria described above can either be used alone or in combination with other criterion when determining if it is suitable or not to calculate battery state of health.